1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus such as an optical disk apparatus and the like for recording/reproducing information by irradiating a recording medium with light beams. More particularly, the invention relates to an optical information recording/reproducing apparatus capable of effectively reducing an offset produced in a tracking error signal because of light diffracted by recording marks on adjacent tracks.
2. Description of the Related Art
An optical information recording/reproducing apparatus for recording/reproducing information by irradiating a recording medium such as a phase change medium with light beams has actually been used as an optical disk apparatus. Information is recorded on an optical disk with a recording mark formed by means of the phase change in the recording film caused by irradiation by light beams. This recorded information is reproduced by detecting a change in intensity of the reflected light beams from the optical disk depending on existence of the recording mark.
In a conventional general optical disk apparatus, recording/reproducing of information is performed only by either a groove or a land on an optical disk. The groove is for guiding light beams and formed on the optical disk along a circumferential direction thereof in a shape of concentric circles or spiral. The land is an area held between grooves having a height different from the groove. Only using either the groove or the land is done for recording/reproducing operations.
In the conventional optical disk, tracks in which recording marks are formed exist only in the groove or the land. In the optical disk on which information is recorded/reproduced only using the groove, adjacent tracks are adjacent grooves sandwiching a land, while in the optical disk having tracks in the lands adjacent lands sandwiching a groove are the adjacent tracks. In these cases, track intervals are equal to land or groove widths.
In recent years, research and development have been directed toward higher density recording in an optical disk apparatus. As a method of this, a land and groove recording system for recording information both in a land and a groove is known (Japanese Patent Application KOKAI Publication No. 57-50330). According to this land and groove recording system, there are almost no track intervals because tracks exist both in the groove and the land and track pitches are very narrow. Reproduction of recorded information is, as in the case of the conventional apparatus, performed by detecting a change in intensity of reflected light beams from an optical disk. Because of the narrow track pitches, however, cross talk due to reflected lights from recording marks on adjacent tracks is a problem. In order to deal with this problem, there has been presented a technique of reducing cross talk by properly devising a structure of the recording medium and a depth of the groove.
Reproduction of recorded information is performed by detecting a change of intensity of reflected light beams from the optical disk as in conventional ones. However, this has the problem of crosstalk caused by the reflection of light from the recording mark on an adjacent track because of a narrow track pit. For the problem of a crosstalk, crosstalk reduction technique by improving a structure of the recording medium and a depth of the groove is proposed.
Tracking control is essential to recording/reproducing of information in the optical disk. This tracking control is effected by detecting a tracking error signal indicating a shift in a position of light beams irradiated on the optical disk from a track center in a track width direction thereof from reflected light beams from the optical disk and by controlling an irradiation position of the light beams irradiated on the optical disk by using this tracking error signal to the track center. Conventionally, as a method of detecting tracking errors, a push-pull method has often been used. In the push-pull method, imbalance, in intensity of diffracted light reflected from the optical disk and occurred by the groove or the like and produced due to a shift in the position of the light beams irradiated on the optical disk from the track center, is detected by obtaining a difference between two output signals from the 2-divided photodetector receiving the reflection light beams from the optical disk.
In the case of the land and groove recording system, however, since a considerable amount of light diffracted by the recording marks on the adjacent tracks leaks to the reflected light beams, an offset may be produced by the recording marks on the adjacent tracks if detection of tracking errors is performed by means of the push-pull method. Thus, influence exerted on the tracking error signal by the recording marks on the adjacent tracks increases by an amount equivalent to narrowing of the track pitches relative to conventional ones. Production of an offset in the tracking error signal due to the light diffracted by the recording marks on the adjacent tracks occurs when the recording marks exist only on the adjacent tracks in one side of tracks during recording/reproducing. The reason is why the light diffracted by the recording marks on only one side of the adjacent tracks enters the reflected light beams from the optical disk. Optimization of the structure of the recording medium and the depth of the groove, mentioned above, may be effective for reducing crosstalk due to the recording marks on the adjacent tracks during reproducing of information. But it will not be effective at all for reducing an offset during detecting of tracking errors.
Apparent from the foregoing, when track pitches are narrow as in the case of the land and groove recording system, an offset is produced in a tracking error signal because of light diffracted by recording marks on adjacent tracks and thus it is difficult to effect tracking control with respect to remaining errors permitted by the system.